I have listed my favorite war movies. However, they may go right over the head of a 12-year old. Taking the age of your son into account, I think the most appropriate movie, or at least the first movie to start with, is the 1930 version of All Quiet on the Western Front. It is a straight-forward telling of what it was like to fight in WWI.
Of particular interest is that the movie starts out with a school teacher persuading his class of high school-age boys (not much older than the age of your son) to enlist in the miliitary for glory, honor, patriotism. etc. The movie's protagonist is one of those boys. The movie records the experiences of this boy during military training and actual combat. Later in the film, the protagonist, now a veteran, returns as a guest to the teacher's classroom and..... (you'll have to see the movie for more). One thing for sure, it shows that war is not glamorous!